While on campaigns board, on the top right side you can select a preferred view: table or list.
If you use the table view simply hover over the relevant campaign and click on the green button to publish it.
If you use the list view, choose the 3 dots icon on the right side of the relevant campaign and click on the publish button.
In the same way, you can pause or schedule your campaigns.
Tip: watch your campaign live right after you publish it.